Patients with unusual chromosomal abnormalities are being studied to determine the phenotypic effects of imbalance of chromosomal material. The frequency of chromosomal abnormalities in select groups of individuals with congenital malformations and/or mental retardation is being determined. Etiologic factors, such as mosaicism, balanced translocations, and other structural abnormalities of the chromosomes, are being sought among the relatives of patients with gross chromosomal abnormalities.